Up events are quite different from Down events. Because of the
asymmetrical nature of web programming and because of the reactive model
used, an Up event must be created on the server and wrapped into a
callback (or something the client can build a callback with).

create param creates an Up event.
If ~name is present, the coservice used to transmit the event will
always have the same name, even if the server is restarted.
~scope describes the visibility of the event. By default, it is
`Site if it is called during initialisation,
`Client_process otherwise.